Sea water contains, as is well-known, apart from sodium chloride, whose solubility increases with increasing temperature, other salts whose solubility decreases with rising temperature. During the heating and concentration of the water in sea water-vaporization installations for the production of potable water, i.e. sea-water desalination plants, these salts tend to reach a supersaturated state and the crystallize out in the form of crusts or scale on the heating surfaces of the heat exchanger.
The main scale formers are calcium carbonate, magnesium hydroxide and calcium sulfate (see CHEMIE ING. TECH., vol. 47, 1974, No. 16, pp. 664-669). Such scale deposition hinders the heat transfer, reduces the flow cross section through the heat exchanger and reduces the efficiency of the apparatus. Furthermore, it causes significant downtime of the installation for cleaning and descaling. Because of this phenomenon, the art makes use of various additives to reduce or prevent the aforementioned scale or crust deposition on the heating surfaces of thermal sea-water desalination plants.
In one such process, acids are added to the water in the plant in stoichiometric quantities, i.e. a stoichiometric quantity sufficient to react with the calcium and magnesium cations. The acid may be sulfuric acid and reacts with the salt solution so that the carbonates form carbon dioxide which can be removed in a subsequent degassing stage, the water being enriched with sulfate anions. The acid feed is problematical because it gives rise to the danger of increased corrosion and can effect the health of operating personnel. The danger is increased in the event of failure of the control systems and inadvertent oversupply of acid. In addition, since it is practical or economical only to use concentrated acid, the handling of the process is dangerous and difficult. In order to overcome these disadvantage, it has been proposed to introduce polyphosphates into such salt solutions. This technique, however, has been found to be practical only for calcium carbonate deposition and, in addition, has been found to be useful at operating temperatures up to 90.degree. C. because of the thermal instability of the polyphosphates.
The use of chelate-forming agents is uneconomical because they must be added in stoichiometric quantities to the solution at high cost.
It has also been proposed to introduce, the substoichiometric quantities, additives in the form of polymers or copolymers, for example, of hydrolyzed polymaleic acid with a molecular weight of 300 to 5000 (see U.S. Pat. No. 3,810,834) or polymers of maleic acid anhydride as well as copolymers containing one or more monoethylenically unsaturated monomers such as acrylic acid. These copolymers or their hydrolyzates, with a molecular weight of 300 to 1000, are described in German open application DT-OS 24 05 192.
Such additives have been found to increase the solubility of scale- and crust-forming salts and/or form with the latter complexes which do not deposit on the heating surfaces of the sea-water desalination plant.
However, the production of such additives is difficult and relatively expensive and the additives are frequently toxic.
Aromatic solvents in which the polymerization must be carried out, require removal with a high degree of completeness so that distillation processes and units are required. When copolymers are produced as described above, the individual monomers must be supplied in precisely metered quantities to avoid the formation of block polymers which are less effective or ineffective.
Moreover, when the monomers used in the copolymerization process are toxic, any traces of nonreacted monomer must be removed, again by distillation of special crystallization processes and units.
Finally, the polymerization is often carried out with peroxide-type catalysts with a host of problems which are characteristic of the use of such catalyst systems.
It has also been proposed to provide additives, which operate in a similar manner to those described previously, from a mixture of sulfuric acid esters and hydrolyzed polysaccharides, carboxymethyl cellulose and sulfonated fatty alcohols and/or fatty acids (see German open application - Offenlegungsschrift DT-OS No. 21 64 240). The additives thus made are less critical but are still more expensive than desirable since the sulfuric acid esters and sulfonated fatty alcohols or acids must be produced separately from one another. The efficiency of these additives at high temperatures against the formation of calcium carbonate deposits still leaves much to be desired.